Work in progress involves the study of the regulation of diphteria toxin production. We have shown that the iron mediated inhibition of diphtheria toxin production is directed at the level of transcription. Mutant analysis has strongly suggested that the regulation of toxin production is mediated by a bacterial specific factor that specifically acts with iron as a negative controlling element. Recent studies have resulted in the isolation and characterization of extragenic suppressor strains of Corynebacterium diphtherae. Analysis of missense proteins resulting from suppression are currently in progress.